I've used both, but unlike most i ended up preferring fixed power scopes.
Variables are what is in demand, so thats what all the technology is going into and thats what sells. If reselling is in your future, go variable.
Scott_Rap posted his experience, which mirrors mine perfectly. My variables rarely ever left 8x. Never once did i put my eye behind the scope and decide fiddling with the magnification was worth the time spent doing it.
Fixed scopes have fewer lenses and fewer moving parts, so for a given price point, if comparing apples to apples, will be clearer, brighter and more durable.
My 10x is sufficient for steel out to 1700+yds. Its never been the magnification at issue.
Of the very few occasions ive wanted a variable magnification, it was always because 10x was a little too much. At a couple schools, they threw in a few 50-100 yard pop-ups with a short exposure time, while shooting longer range targets.
I experimented, and for that purpose, a Burris fastfire mounted to my scope was alot quicker than dialing down the magnification.
Buy a variable if resale is a possibility.
If your on a budget and are going to keep the scope forever, a fixed 10 gives you alot of performance for the money. They tend to be tougher as well.
If only USO would make me an ST8-TPAL!